Effect of SBS compatibilizer on the interphase boundary of polymer blends of polystyrene and natural rubber

Abstract

Microhardness (H) has been used to quantitatively characterize the quality of the interphase boundary of blends of atactic polystyrene (PS) with natural rubber (NR) and the compatibilizer styrene-butadiene-styrene (SBS) block copolymer. Qualitative conclusions are drawn for the same purpose from scanning electron microscopy (SEM) observations on surface indentation. Cast films of neat PS and the blends PS/NR, PS/SBS and PS/NR/SBS were prepared from a common solvent (toluene). While neat PS shows a constant H = 175 MPa, the PS/SBS blend exhibits a gradual decrease starting from H = 130 - 140 MPa at a distance of about 100 μm from the phase boundary up to the final value of 25 MPa. For the PS/NR blend this decrease starts at about 50 μm, reaching the same final value. Owing to the elastic recovery, no indentations on the NR or SBS particles surface can be detected as confirmed by SEM. The compatibilized blend PS/NR/SBS is characterized by the thinnest phase boundary (about 25 μm) and a final H value of 75 MPa which demonstrates that the compatibilizer SBS is distributed just on the phase boundary between PS and NR. Results confirm previous findings in incompatible blends of poly(methyl methacrylate)/NR, highlighting that microindentation is a sensitive tool for studying the breadth and quality of the interphase boundary in compatibilized or non-compatibilized polymer blends and other non-homogeneous materials. This technique allows, in addition, the localization of the compatibilizer in the polymer blends.